1. Field of the Invention
The present invention relates to tilt correction methods of a movable portion, tilt correction methods of an objective lens for an optical disk, and objective lens driving devices for an optical disk. More specifically, the present invention relates to a tilt correction method of a movable portion, tilt correction method of an objective lens for an optical disk, and an objective lens driving device for an optical disk for recording/reproducing information with respect to an optical disk of an optical information recording medium such as an MD, CD-ROM, and DVD.
2. Description of the Background Art
An optical recording/reproducing apparatus collects/scans as small spots of light beams over an information recording track of an optical disk for recording and erasing information. The optical recording/reproducing apparatus reads reflection of the light beam directed to an information recording surface for reproducing information.
When such an optical recording/reproducing apparatus records information, for example, the optical disk which is rotating at high speed may be subjected to surface vibration, decentering, and rotation oscillation. Accordingly, to reliably record, erase and reproduce information with respect to the optical disk, spots of light beams have to precisely follow the track even in the event of surface vibration or the like.
To meet this need, an objective lens for an optical disk used in an optical recording/reproducing apparatus is conventionally provided with a drive control mechanism which includes a focus servo mechanism adapted to slightly move in the perpendicular direction (optical axis direction) with respect to a disk surface, and a tracking servo mechanism adapted to slightly move in the radial direction with respect to the disk surface (orthogonal direction with respect to a recording track). The drive control mechanism has constantly provided positional control of an objective lens for an optical disk.
Now, a brief description of an objective lens driving device for an optical disk provided with a conventional drive control mechanism will be given with reference to the drawings. FIG. 42 is a side view of an objective lens driving device for an optical disk 101 used for explaining a positional relationship between objective lens driving device for optical disk 101 and an optical disk 10 in the conventional art. FIG. 43 shows an objective lens driving device for optical disk 101 when viewed from above.
As shown in FIGS. 42 and 43, objective lens driving device for optical disk 101 is positioned below optical disk 10. The direction perpendicular to the surface of optical disk 10 (optical axis direction) is hereinafter defined as the focus direction Fo (the direction indicated by an arrow Fo in the drawing), the direction parallel to the surface of optical disk 10 and perpendicular to the track direction (radial direction of optical disk 10) as the tracking direction Tr (the direction indicated by an arrow Tr in the drawing), and the tangential direction of the disk as the tangential direction Ta (the direction indicated by an arrow Ta in the drawing).
A reflection mirror 9 is provided on a base for guiding a laser beam to objective lens 1. A movable portion includes: an objective lens 1 of a pickup optical system for an optical disk; an objective lens holder 2 for holding objective lens 1 approximately at the center; a pair of hollow focus coils 6 provided on the side walls of objective lens holder 2; and a tracking coil 7 fixed at each focus coil 6. The movable portion is supported by an elastic supporting member 3 of, for example, four parallel metal wires. Elastic supporting member 3 supports the movable portion in such a way to allow its slight movement in focus direction Fo and tracking direction Tr that are orthogonal to the longitudinal direction of elastic supporting member 3.
Elastic supporting member 3 has one end mounted to a mounting member 8 fixed to the side surface of objective lens holder 2 that is orthogonal to tracking direction Tr, and the other end mounted to a fixed portion 11 (an optical base). Elastic supporting member 3 has, on its one or both surfaces (on both surfaces in FIG. 43), a damper material 12. Note that mounting member 8 is used for mounting elastic supporting member 3 and serves to provide electrical communication among focus coil 6, tracking coil 7 (these coils may be hereinafter collectively referred to as “a driving coil”), and metal elastic supporting member 3.
Objective lens driving device for optical disk 101 is further provided with a magnetic circuit which includes a magnet 4 and a yoke 5 for driving objective lens holder 2 in focus direction Fo and tracking direction Tr. Yoke 5 consists of two inner yokes 5(a) and two outer yokes 5(b) which are orthogonal to the optical axis direction. Two magnets 4 are respectively mounted on the surfaces of two outer yokes 5(b) that face inner yokes 5(a).
Since inner yokes 5(a) are inserted in focus coils 6, objective lens holder 2 fixed to the driving coil can be driven by controlling a current flowing through the driving coil.
Namely, the magnetic circuit formed of magnet 4 and yoke 5 as well as focus coils 6 in the magnetic circuit comprise a dynamoelectric converter which causes movement in focus direction Fo. Thus, by controlling the current in focus coil 6, the driving force in focus direction Fo can be varied. As a result, lens holder 2 mounted with objective lens 1 can be translated in focus direction Fo against the elastic force of elastic supporting member 3.
Further, the magnetic circuit formed of magnet 4 and yoke 5 as well as tracking coil 7 positioned in the magnetic circuit comprise a dynamoelectric converter which causes movement in tracking direction Tr. Thus, by controlling the current in tracking coil 7, the driving force in tracking direction Tr can be varied. As a result, lens holder 2 mounted with objective lens 1 can be translated in tracking direction Tr against the elastic force of elastic supporting member 3.
Here, as a prior art elastic supporting member, an objective lens driving device disclosed in Japanese Patent Laying-Open No. 6-139599 (Japanese Patent No. 2981351) is illustrated. FIG. 44 is a diagram an elastic supporting member (elastic material) of the objective lens driving device when viewed from above. Linear portions 3a and 3b, respectively extending from the movable and fixed portion sides of elastic supporting member 3, are not collinear. Elastic supporting member 3 has a bent portion 3c between linear portions 3a and 3b (elastic supporting member 3 is bent in tracking direction Tr).
A damper material 12 is fixed to connect an arm portion 3d branching from linear portion 3a extending from the movable portion side and a protruding portion 3e. Elastic supporting member 3 is in the form of a leaf spring with a damper material that has a damping effect applied on one or both surfaces.
The bent portion and damper material of the elastic supporting member serve to suppress vibration of the movable portion in focusing direction Fo and tracking direction Tr and suppress expansion/contraction or torsional oscillation in the longitudinal direction. Thus, a resonance peak is restrained to provide stable driving control.
However, the above described prior art objective lens driving device for the optical disk suffers from the following problems.
The above described prior art objective lens driving device for optical disk tends to deform (expand/contract) in the longitudinal direction because of the bent portion of elastic supporting member 3. For example, as shown in schematic side views of FIGS. 45A, 45B, and 45C, when objective lens 1 is vertically moved in focus direction Fo, it tilts in a specific direction. Namely, the optical axis (solid lines in the drawings) of objective lens 1 are inclined.
FIG. 45B shows that the movable portion is in a neutral position and the optical axis is not inclined. FIG. 45A shows that objective lens 1 is moved upwardly in focus direction Fo. FIG. 45C shows that objective lens is moved downwardly in focus direction Fo. As shown in FIG. 45A, when objective lens 1 is moved upwardly in focus direction Fo, the optical axis is inclined toward fixed portion 11 (+side) in the Ta direction (tangential direction) in the drawing. As shown in FIG. 45C, when objective lens 1 is moved downwardly in focus direction Fo, the optical axis is inclined toward the side opposite fixed portion 11 (−side) in the Ta direction (tangential direction) in the drawing.
If objective lens 1 is inclined as described above, deflection of elastic supporting member 3 produces a moment on the movable portion. For example, if the supporting interval of four parallel elastic supporting members 3 (having a length of 11.45 mm) is 8.26 mm in width and 2.91 mm in height and objective lens 1 is vertically moved by 0.4 mm in focus direction Fo, when elastic supporting member 3 has a bent portion, a tilt amount would be about ±7.6′ in the Ta direction. On the other hand, if the elastic supporting member has an linear shape without any bent portion (in this case deformation is unlikely to occur in the longitudinal direction), the tilt is caused by deformation of elastic supporting member 3, and hence the tilt would be no more than about ±0.4′ in the Ta direction.
Further, the tilt is affected by the supporting interval of elastic supporting member 3. FIG. 46 is a graph showing a relationship between the interval of the elastic supporting members in the height direction and a tilt amount of the optical axis of the objective lens when only the interval in the height direction is varied in the case of the above described elastic supporting member 3. It also represents the tilt amount of the optical axis when objective lens 1 is moved upwardly in focus direction Fo by 0.4 mm. As can be seem from FIG. 46, the smaller the interval between the elastic supporting members in the height direction is, the greater the tilt amount of the objective lens optical axis is. As the movable portion is reduced in size and thickness, the interval of the elastic supporting members in the height direction decreases and the tilt amount of the objective lens optical axis increases, whereby the problem becomes more serious.
In recent years, the amount of information that an optical recording/reproducing apparatus is required to process is rapidly increased. It is desired that a recording surface density for optical recording is considerably increased accordingly. The recording surface density can be increased for example by reducing the wavelength of a light source or by providing an objective lens with greater numerical aperture. As to the former method of reducing the wavelength of the light source in the optical recording/reproducing apparatus, although light sources having a wavelength of about 780 nm or 650 nm are primarily used, the usage of light sources is gradually shifting to those of violet or blue having a wavelength of about 400 nm.
To mention the effect of coma aberration, since coma aberration is in inverse proportion to the wavelength of a light source, it increases with reduction in the wavelength of the light source. Accordingly, to reduce coma aberration, a tilt amount on the side of the optical recording/reproducing apparatus must be reduced to about 50–60% of the current amount. With the increasing need for reducing the tilt amount in the optical recording/reproducing apparatus, the tilt amount allowed to an actuator is desirably 50% or lower of the current amount. For example, if the tilt amount of optical axis of the objective lens when the objective lens is vertically moved in focus direction Fo is currently allowed to have about ±7.6′ in the Ta direction (with light source wavelength of 780 nm), in the case of a optical recording/reproducing apparatus with a light source wavelength of 410 nm, a tilt amount must be restrained to about ±4′.